1. Field of the Invention
The invention relates to a Coriolis flow sensor comprising a loop-shaped Coriolis tube mounted in a housing with two ends lying next to one another, said ends being fixed in a tube fixation means, while the portion of the tube located between said ends lies free from the housing, which flow sensor comprises excitation means for causing the tube to oscillate about an excitation axis as well as detection means for detecting displacements of portions of the tube during operation.
2. Description of the Related Art
Such a flow sensor having a loop-shaped Coriolis tube is known from EP 1 719 982 A1. Various types of loop-shaped Coriolis tubes are described therein, both of the single loop type and of the (continuous) double loop type. The present invention relates to any of these types, but is not restricted thereto.
A Coriolis flow sensor (or Coriolis flow sensor system) comprises at least one vibrating tube, often denoted Coriolis tube, flow tube, or sensing tube. This tube or these tubes is or are fastened at both ends to the housing of the instrument. These tube ends serve at the same time as feed and discharge ducts for the liquid or gas flow to be measured.
Besides the flow tube (or tubes), a Coriolis flow sensor comprises two further subsystems, i.e. one for excitation and one for detection. The excitation system (exciter) brings the tube into vibration. For this purpose, one or several forces or torques are applied to portions of the tube. The detection system usually detects the displacements of one or several points of the tube as a function of time. Instead of this displacement, the force (or torque) exerted by the tube on its environment may alternatively be measured; what will be described below with reference to displacement detection is equally valid for force detection.
The same two placement alternatives are possible for excitation and detection. One is to have the excitation and detection take place between the housing and the tube. The other one is to have the excitation or detection take place between different points or sections of the moving flow tube or—if the instrument has several tubes—between the individual tubes.
In the case of a Coriolis flow sensor (also referred to as “the (sensor) instrument” or “the flowmeter” hereinafter) designed for measuring small flows, it is desirable for the entire tube to lie in one plane both for the purpose of the measuring accuracy and for the purpose of ease of manufacture.
The vibration of the tube generated by the exciter takes place at a more or less fixed frequency which varies slightly only as a function of the density of the medium flowing through the tube. The vibration frequency is almost always a natural frequency of the tube so that a maximum amplitude can be achieved with a minimum energy input.
The invention is based on the recognition that two vibration problems may arise as a result of the vibration of the tube if no additional measures are taken.
The first problem may arise when two identical instruments are located close to one another while their vibration frequencies substantially coincide. One instrument may then excite the other instrument via the housing and the supporting surface, in general just outside its natural frequency, with a phase that will practically always differ from that of its own excitation. This is a real problem, because in practice, for example in mixing processes, two, three, or sometimes up to twenty flowmeters are arranged next to one another. It is found then that the measuring results can vary with a certain periodicity independently of the flow.
A second problem is the sensitivity to own vibrations: when a Coriolis flowmeter is placed on a non-rigid surface, for example a thin plate, or in a system of ducts, said surface may start to vibrate along with the flowmeter. The own vibrations are seen as shifts in the zero point. The accuracy of the sensor, and thus of the measurement, is influenced thereby in an unpredictable manner.